Thermochemical processing of used wooden railroad ties recovers wood preservatives and produces fuels that can be used to make new ties.

Credit: Shutterstock

Thermochemical processing of used wooden railroad ties recovers wood preservatives and produces fuels that can be used to make new ties.

Credit: Shutterstock

A little-known fact: Some 21 million wooden railroad ties are taken out of service in the U.S. each year. What happens to them? Many of the used ties, which on average are 30 years old, are burned to produce heat and electricity, and the remainder are recycled as landscaping timbers or disposed of in landfills, according to Nicole Labbé, Pyoungchung Kim, and their colleagues at the University of Tennessee’s Center for Renewable Carbon. However, changing environmental rules on burning and reusing the chemically treated wood combined with the low cost of natural gas has meant railroad ties are increasingly being landfilled. This situation led the Tennessee researchers to work with wood-preservative company Nisus to develop a commercial-scale thermochemical process that increases the value of used railroad ties and reduces the environmental impact of disposing of them (ACS Sustainable Chem. Eng. 2017, DOI: 10.1021/acssuschemeng.7b02666). The process involves chipping the ties and heating the material to extract creosote or copper naphthenate, which are commonly used preservatives added to railroad ties to ward off fungal invaders that degrade wood. The cleaned-up wood can be burned as boiler fuel or preferably can be pyrolyzed to create a bio-oil that can be converted to gasoline, diesel fuel, and chemicals. The leftover solid material, known as biochar, can be burned like coal or added to farmland to improve soil quality and sequester carbon. The researchers suggest the recovered preservatives and the produced fuel and chemicals can be directed back into preparing new railroad ties. “We have demonstrated that our approach can make a better use of the old ties, both economically and environmentally,” Labbé says.